February 28, 2014

Even though climate scientists claim that the rise in average global temperatures has slowed over the past 10 or 20 years, research published this week in the journal Nature Climate Change has found a continued increase in extreme hot temperatures during that time.

In the study, Dr. Lisa Alexander of the University of New South Wales (UNSW) in Australia and an international team of colleagues discovered a dramatic increase in both the number and area of extremely hot land temperatures, despite the supposed global warming hiatus.

“It quickly became clear, the so-called ‘hiatus’ in global average temperatures did not stop the rise in the number, intensity and area of extremely hot days,” Dr. Alexander explained in a statement Wednesday. “Our research has found a steep upward tendency in the temperatures and number of extremely hot days over land and the area they impact, despite the complete absence of a strong El Niño since 1998.”

She and her fellow researchers opted to examine the extreme end of the temperature spectrum, as that region is where the effects of global warming tend to have the earliest and deepest impact on society – as demonstrated by the extreme conditions experienced by inhabited areas in Australia over the past two summers.

Their efforts revealed that, on average, extremely hot events were impacting more than twice the area than they did 30 years ago. Dr. Alexander’s team reviewed data pertaining to hot days dating back more than three decades, comparing temperatures of every day of the year to the same calendar days from 1979 through 2012.

The investigators selected the hottest 10 percent of all days over that period and classified them as the hot temperature extremes. They found that from 1979 through 2012, regions that experienced 10, 30 or 50 extremely hot days above that average experienced the greatest increase in number of extremely hot days and the surface area covered by that heat. Furthermore, those trends continued through the supposed hiatus that started in 1998.

“Our analysis shows there has been no pause in the increase of warmest daily extremes over land and the most extreme of the extreme conditions are showing the largest change,” said postdoctoral climate scientist Dr. Markus Donat.

He added that the researchers also found that “regions that normally saw 50 or more excessive hot days in a year saw the greatest increases in land area impact and the frequency of hot days. In short, the hottest extremes got hotter and the events happened more often.”

The study authors said their findings indicate that, even though global annual average near-surface temperatures are widely used to gauge global warming, they do not account for all parts of the climate system. Even if global annual mean temperatures grow stagnant for a period of 10 to 20 years, it does not necessarily mean that warming has stopped – other factors, such as extreme temperatures, must also be considered.

“It is important when we take global warming into account, that we use measures that are useful in determining the impacts on our society,” explained lead researcher Sonia Seneviratne, a professor with the ETH Zurich Institute for Atmospheric and Climate Science.

“Global average temperatures are a useful measurement for researchers but it is at the extremes where we will most likely find those impacts that directly affect all of our lives,” she added. “Clearly, we are seeing more heat extremes over land more often as a result of enhanced greenhouse gas warming.”

Image 2 (below): This image shows a time series of temperature anomalies for hot extremes over land (red) and global mean temperature (black, blue). The anomalies are computed with respect to the 1979-2010 time period. The time series are based on the ERA-Interim 95th percentile of the maximum temperature over land (Txp95_Land, red) and the global (ocean + land) mean temperature (Tm_Glob) in ERA-Interim (blue) and HadCRUT4 (black). Credit: Nature Climate Change commentary by authors: No pause in the increase of hot temperature extremes